Pressure sensitive adhesives have significant commercial application. While a variety of adhesives have been prepared from countless combinations of ingredients including various polymers, e.g., acrylates, (meth)acrylates; epoxies; crosslinkers; heat and cationically curable polymers; and heat or radiation sensitive initiators; not all such compositions are suitable for use, especially in optical applications where clarity and stability are required. A useful adhesive for optical applications should be optically clear, and should also maintain that clarity over the life of the product in which it is used. The adhesive should withstand conditions present during manufacturing of optical elements and during use of such elements without losing a substantial amount of clarity. This property involves the adhesive being stable on its own, and also being stable and compatible when used with other elements in an optical element or optical product.
Optical elements include components bonded together by the adhesive. The adhesives are used, for example, to bond materials to polymeric materials such as polyesters; to bond materials to rigid materials such as rigid polycarbonate, polymethyl methacrylate, or glass; to bond materials to polarizer layers; etc.
Often, any of these components of an optical element, can adversely affect the stability, clarity, bond strength, or other performance property of an adhesive in the same optical element. Polyearbonates, for example, are known to outgas in response to changing environmental conditions such as heat and humidity, producing bubbles or partial or full delamination at the adhesive bond between the polycarbonate and another layer of an optical element. Bubbling and delamination can be particularly common when the outgassing layer is bonded to another layer or laminate that exhibits low vapor transmissivity. Bubbles and delamination can affect clarity and integrity of the optical element, and should be avoided. It can be preferred, therefore, in these and other settings, that an adhesive be stable and not delaminate, bubble, or lose its clarity or integrity during use.